This invention relates to the selective dissolution of metal compounds from metal surfaces and more particularly to a method of determining the proportion of chemically combined nickel in a finely porous, permeable, metal membrane.
In recent years there has been considerable interest in the use of diffusional separation processes for the separation of the components of difficultly separable gaseous mixtures and in the development of suitable finely porous permeable metal membranes for use in such processes. In order to operate satisfactorily as a diffuser-separator such membranes must have a multitude of exceedingly fine pores i.e. pores so small as to be beyond the resolving power of the light microscope. Membranes of this character may be used with advantage to separate the components of gaseous mixtures that comprise or consist of the isotopes of an element; for example, they may be used to separate the isotopic species of uranium hexafluoride U235F6 and U238F6 or the isotopes of chlorine Cl235 and Cl237.
Then finely porous metal diffuser membranes are used in separating gaseous mixtures containing or consisting of highly corrosive gases such as uranium hexafluoride a number of problems arise due to the tendency of the corrosive gas to attack the constituent metal of the membrane. Because of its finely porous character the membrane has a very large total surface area exposed to the gas being separated and thus it is easily possible for chemical attack to proceed to the point where the fine pores are substantially completely plugged with corrosion products and the membrane rendered useless for its intended purpose.
In the case of uranium hexafluoride the chemical attack of the gas on the membrane may be reduced by forming the membrane of a relatively resistant metal such as nickel or copper and may be retarded by protecting the membrane with, for example, elemental fluorine to form a protective coating of nickel fluoride on the membrane surfaces under controlled conditions. However, even under optimum conditions the hexafluoride gas continues to attack the membrane and eventually the pores of the membrane become plugged. It is thus desirable in connection with such diffusion processes to have a method of determining the extent to which the membrane has been attacked at any given time and the present invention provides a method whereby the metal compounds formed by the action of the hexafluoride gas on the constituent metal of the membrane may be removed from the membrane and analyzed to determine the proportion of combined metal present in the membrane.
It is accordingly an object of the present invention to provide a method for determining the extent to which a finely porous metal membrane has been attacked by a corrosive gas to which it has been exposed.
It is a further object of the invention to provide an improved method for selectively removing nickel compounds from nickel surfaces without dissolving appreciable amounts of metallic nickel.
It is still another object of the invention to provide an improved method of dissolving nickel compounds from the pores of a finely porous nickel membrane so that the proportion of combined nickel in the membrane can be determined.
Other objects of the invention will be in part obvious and in part pointed out hereinafter.
As conducive to a clearer understanding of the present invention it may be pointed out that when a finely porous, permeable nickel membrane is used in the diffusion separation of the isotopic species of uranium hexafluoride, the hexafluoride tends to react with the constituent metal of the membrane to form nickel fluoride and the hexafluoride is reduced to the tetrafluoride. The extent to which this reaction has proceeded may be determined by removing the nickel fluoride from a given portion of the membrane and determining by standard methods of chemical an analysis the quantity of combined nickel that was associated with the given portion of membrane. However, the problem of removing nickel fluoride quantitatively from such a nickel membrane in order to make an accurate analysis is a difficult one, particularly in view of the finely porous character of the structure. Nickel fluoride is somewhat soluble in water but if an attempt is made to leach out the nickel fluoride with water two difficulties are encountered. On the one hand the finely porous character of the membrane makes it difficult for the water to reach the nickel fluoride and this fact coupled with the fact that the solubility of nickel fluoride in water is relatively low, results in a rate of dissolution that is impractically slow where quantitative removal is important. On the other hand the nickel fluoride tends to hydrolyze in water to form hydrofluoric acid which in turn reacts with the nickel of the membrane to form more nickel fluoride. To the extent that such additional nickel fluoride is formed the analysis will be in error.
I have discovered that the foregoing difficulties may be avoided and quantitative and highly selective removal of nickel fluoride and other nickel compounds from a nickel membrane may be achieved by using as a dissolving agent an aqueous solution of ammonium hydroxide that is substantially free from uncombined oxygen. Such a solution rapidly dissolves nickel fluoride from the fine pores of the membrane, the rate of solution being presumably accelerated by the formation of nickel-ammonium ions. It has been further found that if the ammonia solution is carefully purified to remove free oxygen the tendency of the solution to attack the metal of the membrane is negligible and the removal of nickel fluoride is quantitative. The nickel content of the resulting solution may be determined by the usual gravimetric method or by other known standard methods of analysis.